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Robin Roberts is celebrating more than just the beginning of a new year.

After undergoing a bone marrow transplant back in September, the "Good Morning America" co-anchor has hit a "major milsetone" in her recovery and tweeted a picture of herself wearing a Santa hat holding a calendar with the caption: "Every morning I mark the day post my bone marrow transplant. Today I reached a major milestone...Day 100! Blessings..XO."

Roberts also posted a message on her Facebook, thanking fans for their support. "I couldn't let the year end without saying...thank you. Thank you for your prayers, positive energy and well wishes," she said. "My sister, Dorothy, always comes up with a family motto for the new year. This year's is: "All will be serene in 2013."

The 100 day celebration was even sweeter with the return of K.J., Roberts' beloved Jack Russell terrier. "Look who made it back for my 100 day celebration...KJ!!" Roberts tweeted on Dec. 29. "Can't believe she's finally home."

The day Roberts underwent the transplant, her doctor Dr. Gail Roboz appeared on Good Morning America and pointed out the significance of the 100 day mark. "We have to watch patients super closely for 100 days. We don't trust anything for the first 100 days. After that, we're happy."